The repair of traumatised structures on and in an eye and its adnexa is often troublesome. The use of sutures in e.g. the cornea is causing discomfort, deformations, distortions and may impair the visual acuity. Other eye components, such as the iris, lens structures and the retina are difficult or hardly possible to suture or to join with clips and related aids. Furthermore, sutures and clips do inevitably induce foreign body reactions and scar formation. The positioning of an implant in ocular structures, e.g. a partial-thickness or a penetrating keratoplasty or a prosthesis in the cornea, requires the use of elaborated techniques including the use of haptics to keep it retained in proper position, which may lead to irritations and adverse reactions. Accordingly, current methods for repairing structures on, at, and in an eye are associated with discomfort and the possibility of inducing permanent damage to the visual acuity. The use of a composition enabling structures with wet surfaces to be attached in desired position, remaining adherent for a predicted time period without causing any opacities in optically important components, or any deformation, scars or unacceptable foreign body reactions would therefore be highly desirable.
Polyphenolic proteins, preferentially isolated from mussels, are known to act as adhesives. Examples of such proteins can be found in e.g. U.S. Pat. No. 4,585,585. Their wide use as adhesives has been hampered by problems related to the purification and characterisation of the adhesive proteins in sufficient amounts. Furthermore, the requirement for biocompatible suitable cross-linkers and other additives have limited their use. Chemicals, such as bifunctional conjugating compounds, and enzymes are commonly associated with toxic reactions and other biomedical side effects. Additionally, it is difficult to extensively purify enzymes with retained high activity, avoiding denaturation and adverse effects on cells, tissues or organs.
Mussel adhesive protein (MAP) is formed in a gland in the foot of byssus-forming mussels, such as the common blue mussel (Mytilus edulis). The molecular weight of MAP from Mytilis edulis is about 130.000 Dalton and it has been disclosed to consist of 75-80 closely related repeated peptide sequences. The protein is further characterised by its many epidermal growth factor like repeats. It has an unusual high proportion of hydroxy-containing amino acids such as hydroxyproline, serine, threonine, tyrosin, and the uncommon amino acid 3,4 dihydroxy-L-phenylalanine (Dopa) as well as lysine. It may be isolated either from natural sources or produced biotechnologically. U.S. Pat. No. 5,015,677 as well as U.S. Pat. No. 4,585,585 disclose that MAP has very strong adhesive properties after oxidation and polymerisation, e.g. by the activity of the enzyme tyrosinase, or after treatment with bifunctional reagents. It is very important in biomedical applications of an adhesive and coating composition to use bioacceptable and biodegradable components, which furthermore should not per se or due to contamination induce any inflammation or toxic reactions. Fillers, including collagens and polysaccharides, have been added to improve the mechanical properties in cases when MAP was used to bond tissues and structures together, further adding to the risk for immunological reactions.
It is also previously known that it is possible to use adhesive compositions based on MAP for ophthalmic purposes. Robin et al., Refractive and Corneal Surgery, vol. 5, p. 302-306, and Robin et al., Arch. Ophthalmol., vol. 106, p. 973-977, both disclose MAP-based adhesives comprising an enzyme polymiser. U.S. Pat. No. 5,015,677 also describes a MAP-based adhesive containing a cross-linking agent and optionally a filler substance and a surfactant. Preferred cross-linking agents according to U.S. Pat. No. 5,015,677 are enzymatic oxidising agents such as catechol oxidase and tyrosinase, but sometimes also chemical cross-linking agents such as glutaraldehyde and formaldehyde. Examples of fillers are proteins, such as collagen and albumin, and polymers comprising carbohydrate moieties, such as chitosan and hyaluronan. U.S. Pat. No. 5,030,230 also relates to a bioadhesive comprising MAP, mushroom tyrosinase (cross-linker), SDS (sodium dodecyl sulfate, a surfactant) and collagen (filler). The bioadhesive is used to adhere a cornea prosthesis to the eye wall.
A major problem associated with known MAP-based bioadhesive compositions, despite the superior properties of MAP per se, is that some constituents, in particular the presently used cross-linking agents, can harm and/or irritate living tissue and cause toxic and immunological reactions. Chemical crosslinking agents, such as glutaraldehyde and formaldehyde, are generally toxic to humans and animals, and it is highly inappropriate to add such agents to a sensitive tissue, such as the eye. Enzymes, such as catechol oxidase and tyrosinase, are proteins, and proteins are generally recognised as potent allergens, especially in case they originate from a species other than the patient. Because of their oxidising and hydrolysing abilities, they can also harm sensitive tissue. Despite these serious drawbacks associated with the presently used cross-linkers, it has been regarded as necessary to include them in order to obtain sufficient curing of the bioadhesive.
Accordingly, there is a need for a MAP-based bioadhesive composition which overcomes these drawbacks and hence, does not harm or irritate sensitive tissues such as the eye.